IMPROVED ZEOLITE-CONTAINING OXIDATION CATALYST AND METHOD OF USE

Abstract: Oxidation catalyst compositions for treating diesel exhaust include ceria and optionally, alumina, each having a surface area of at least about 10 m<2>/g, and a zeolite, e.g., Beta zeolite. Optionally, platinum may be included in the catalytic material, preferably in amounts which are sufficient to promote some gas-phase oxidation of carbon monoxide ("CO") and hydrocarbons ("HC") but which are limited to preclude excessive oxidation of SO2 to SO3. Alternatively, palladium in any desired amount may be included in the catalytic material. The zeolite is optionally doped, e.g., ion-exchanged, with one or more of hydrogen, a platinum group metal or other catalytic metals. The catalyst compositions may be used in a method to treat diesel engine exhaust by contacting the hot exhaust with the catalyst composition to promote the oxidation of gas-phase CO and HC and of the volatile organic fraction component of particulates in the exhaust.

Description: IMPROVED ZEOLITE-CONTAINING OXIDATION CATALYST AND METHOD OF USE CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of appli¬ cation Serial No. 07/973,461, filed November 19, 1992. BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a catalyst composition and method for the oxidation of oxidizeable components of die¬ sel engine exhaust, and more specifically to the treatment of such diesel exhaust to reduce the content of particu- lates and other pollutants discharged to the atmosphere. Background and Related Art As is well-known, diesel engine exhaust is a hetero¬ geneous material which contains not only gaseous pollut- ants such as carbon monoxide ("CO") and unburned hydrocar¬ bons ( "HC") , but also soot particles which comprise both a dry, solid carbonaceous fraction and a soluble organic fraction. The soluble organic fraction is sometimes re¬ ferred to as a volatile organic fraction ("VOF"), which terminology will be used herein. The VOF may exist in diesel exhaust either as a vapor or as an aerosol (fine droplets of liquid condensate) depending on the temper¬ ature of the diesel exhaust. Oxidation catalysts comprising a platinum group metal dispersed on a refractory metal oxide support are known for use in treating the exhaust of diesel engines in order to convert both HC and CO gaseous pollutants and particu¬ lates, i.e., soot particles, by catalyzing the oxidation of these pollutants to carbon dioxide and water. One problem faced in the treatment of diesel engine exhaust is presented by the presence of sulfur in diesel fuel. Upon combustion, sulfur forms sulfur dioxide and the oxidation catalyst catalyzes the S0 2 to S0 3 ("sulfates") with subse- quent formation of condensible sulfur compounds, such as sulfuric acid, which condense upon, and thereby add to, the mass of particulates. The sulfates also react with activated alumina supports to form aluminum sulfates, which render activated alumina-containing catalysts inac¬ tive. In this regard, see U.S. Patent 4,171,289 at column 1, line 39 et seq. Previous attempts to deal with the sulfation problem include the incorporation of large amounts of sulfate-resistant materials such as vanadium oxide into the support coating, or the use of alternative support materials such as α-alumina (alpha), silica and titania, which are sulfate-resistant materials. The prior art also shows an awareness of the use of zeolites, including metal-doped zeolites, to treat diesel exhaust. For example, U.S. Patent 4,929,581 discloses a filter for diesel exhaust, in which the exhaust is con¬ strained to flow through the catalyst walls to filter the soot particles. A catalyst comprising a platinum group metal-doped zeolite is dispersed on the walls of the fil- ter to catalyze oxidation of the soot to unplug the fil¬ ter. As is well-known in the art, catalysts used to treat the exhaust of internal combustion engines are less effec¬ tive during periods of relatively low temperature opera- tion, such as the initial cold-start period of engine op¬ eration, because the engine exhaust is not at a tempera¬ ture sufficiently high for efficient catalytic conversion of noxious components in the exhaust. To this end, it is known in the art to include an adsorbent material, which may be a zeolite, as part of a catalytic treatment system in order to adsorb gaseous pollutants, usually hydrocar¬ bons, and retain them during the initial cold-start per¬ iod. As the exhaust gas temperature increases, the ad¬ sorbed hydrocarbons are driven from the adsorbent and sub- jected to catalytic treatment at the higher temperature. In this regard, see for example U.S. Patent 5,125,231 which discloses (columns 5-6) the use of platinum group metal-doped zeolites as low temperature hydrocarbon ad- sorbents as well as oxidation catalysts. SUMMARY OF THE INVENTION Generally, in accordance with the present invention, there is provided a catalyst composition and a method for oxidizing oxidizeable components of diesel engine exhaust in which at least some of a volatile organic fraction of the diesel exhaust is converted to innocuous materials, and in which gaseous HC and CO pollutants may also be sim- ilarly converted. The objectives of the invention are at¬ tained' by an oxidation catalyst comprising a catalytic ma¬ terial comprising a mixture of high surface area ceria, a zeolite and, optionally, a high surface area alumina. The catalytic material optionally may carry a low loading of platinum catalytic metal dispersed thereon or palladium catalytic metal dispersed thereon. Alternatively, or in addition, the zeolite of the catalyst composition may be doped, e.g., ion-exchanged, with a catalytic moiety such as one or more of hydrogen ion, platinum, copper, nickel, cobalt, iron, etc. The method of the invention is at¬ tained by flowing a diesel engine exhaust, e.g., the ex¬ haust of a diesel-powered automobile or light truck, into contact under oxidation reaction conditions with a cata¬ lyst composition as described above. Specifically, in accordance with the present inven¬ tion there is provided a catalyst composition for treating a diesel engine exhaust stream containing a volatile or¬ ganic fraction, which composition comprises a refractory carrier on which is disposed a coating of a catalytic ma- terial comprising a catalytically effective amount of cer¬ ia and, optionally, a catalytically effective amount of alumina, each having a BET surface area of at least about 10 m 2 /g, preferably a surface area of from about 25 m 2 /g to 200 m 2 /g, and a zeolite, for example. Beta zeolite or a zeolite selected from the group consisting of Y-zeolite, pentasil (e.g., ZSM-5), Mordenite, and mixtures thereof. In one aspect of the present invention, the zeolite comprises a three-dimensional zeolite characterized by pore openings whose smallest cross-sectional dimension is at least about 5 Angstroms and having a silicon to alu¬ minum atomic ratio ("Si:Al atomic ratio") of greater than 5, e.g., a Si:Al atomic ratio of from about 5 to 400. In another aspect of the invention, the zeolite com¬ prises from about 10 to 90, preferably from about 20 to 70, percent by weight, the alumina comprises from about 60 to 5, preferably from about 50 to 20, percent by weight, and the ceria comprises from about 60 to 5, preferably from about 50 to 20, percent by weight, of the combined weight of the zeolite, the alumina and the ceria. Yet another aspect of the invention provides for the zeolite to be doped with a catalytic moiety, e.g., ion-ex¬ changed or impregnated, with an ion or with a neutral met- al-containing species selected from the group consisting of one or more of hydrogen, platinum, rhodium, palladium, ruthenium, osmium, iridium, copper, iron, nickel, chromium and vanadium, preferably, one or both of platinum and iron. Still another aspect of the invention provides that the refractory carrier has a plurality of parallel exhaust flow passages extending therethrough and defined by pas¬ sage walls on which the catalytic material is coated, and further comprising either dispersed platinum carried on the catalytic material in an amount of from about 0.1 to about 60, e.g., 0.1 to 15, preferably 0.1 to 5, g/ft 3 or dispersed palladium carried on the catalytic material in a quantity of from about 0.1 to 200, preferably 20 to 120, g/ft 3 . In accordance with the method aspect of the present invention, there is provided a method for treating a die¬ sel engine exhaust stream containing a volatile organic fraction, the method comprising contacting the stream with any of the catalyst compositions described above under ox- idizing conditions including a temperature high enough to catalyze oxidation of at least some of the volatile organ¬ ic fraction. For example, the temperature of the exhaust stream initially contacted with the catalyst composition may be from about 100°C to 800°C. DEFINITIONS As used herein and in the claims, the following terms shall have the indicated meanings. The term "BET surface area" has its usual meaning of referring to the Brunauer, Emmett, Teller method for de¬ termining surface area by N 2 adsorption. Unless otherwise specifically stated, all references herein to the surface area of a ceria, alumina or other component refer to the BET surface area. The term "activated alumina" has its usual meaning of a high BET surface area alumina, comprising primarily one or more of γ-, θ- and δ-aluminas (gamma, theta and delta). The term "catalytically effective amount" means that the amount of material present is sufficient to affect the rate of reaction of the oxidation of pollutants in the ex¬ haust being treated. The term "inlet temperature" shall mean the tempera- ture of the exhaust, test gas or other stream being treat¬ ed immediately prior to initial contact of the exhaust, test gas or other stream with the catalyst composition. The term "doped" used to refer to a zeolite being doped with a metal or hydrogen, and the terms "dope" or "doping" used in the same context, means that the metal or hydrogen moiety is incorporated within the pores of the zeolite, as distinguished from being dispersed on the sur¬ face of the zeolite but not to any significant degree within the pores of the zeolite. Doping of a zeolite is preferably carried out by known ion-exchange techniques in which a zeolite is repeatedly flushed with a solution con¬ taining metal cations (or an acid to provide hydrogen ions), or the zeolite pores are flooded with such solu¬ tion. However, the defined terms include any suitable technique for incorporating a catalytic moiety, e.g., one or more metals as ions or neutral metal-containing species or hydrogen ions, within the pores of the zeolite, espe¬ cially by exchange or replacement of cations of the zeo- lite . The term "washcoat" refers to a thin, adherent coat¬ ing of a material, such as the catalytic material of the present invention, disposed on the walls forming the par- allel gas flow passages of a carrier, which is typically made of a refractory material such as cordierite or other oxide or oxide mixture, or a stainless steel. Reference herein or in the claims to ceria or alumina being in "bulk" form means that the ceria or alumina is present as discrete particles (which may be, and usually are, of very small size, e.g., 10 to 20 microns in diame¬ ter or even smaller) as opposed to having been dispersed in solution form into another component. For example, the thermal stabilization of ceria particles (bulk ceria) with alumina as described in U.S. Patent 4,714,694, results in the alumina being dispersed into the ceria particles and does not provide the dispersed alumina in "bulk" form, i.e., as discrete particles of alumina. The abbreviation "TGA" stands for thermogravimetric analysis, which is a measure of the weight change (e.g., weight loss ) of a sample as a function of temperature and/or time. The abbreviation "DTA" stands for differen¬ tial thermal analysis, which is a measure of the amount of heat emitted (exotherm) or absorbed (endotherm) by a sam- pie as a function of temperature and/or time. BRIEF DESCRIPTION OF THE DRAWINGS Figures 1-5 show graphically the results of the en¬ gine test of the catalysts of Example 1, set forth numer- ically in TABLE I-B: specifically; Figure 1 is a plot of gas-phase HC conversion for sample E-l and comparative samples C-l, C-2 and C-3 as a function of catalyst inlet temperature; Figure 2 is a plot of carbon monoxide conversion for samples E-l and C-3 as a function of catalyst inlet tem¬ perature; Figure 3 is a plot of VOF removal for samples E-l and C-3 as a function of catalyst inlet temperature; -1- Figure 4 is a plot of the reduction of the total mass of particulates (TMP) for samples E-l and C-3 as a func¬ tion of catalyst inlet temperature; Figure 5 is a plot of SO. and H 2 0 emissions for sam- pies E-l and C-3 and for untreated exhaust as a function of catalyst inlet temperature; Figures 6-9 are plots of hydrocarbon, S0 2 and CO con¬ version for samples E-2, E-3 and C-4 of Example 2: speci¬ fically; Figure 6 is a plot of gas-phase hydrocarbon conver¬ sion as a function of catalyst inlet temperature at 50,000 space velocity for samples E-2, E-3 and C-4; Figure 7 is a plot of gas-phase hydrocarbon conver¬ sion as a function of catalyst inlet temperature at 90,000 space velocity for samples E-2, E-3 and C-4; Figure 8 is a plot of S0 2 conversion as a function of catalyst inlet temperature at 90,000 space velocity for samples E-2, E-3 and C-4; Figure 9 is a plot of CO conversion as a function of catalyst inlet temperature at 90,000 space velocity for samples E-2, E-3 and C-4; and Figure 10 is an exemplary TGA/DTA plot of sample E-8 of Example 5. DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS THEREOF The present invention provides an oxidation catalyst composition which is effective for treating diesel engine exhaust, particularly with regard to reducing the total particulates and HC and CO content of the exhaust. The carbonaceous particulates ("soot") component of diesel en¬ gine exhaust is, as discussed above, known to be comprised of relatively dry carbonaceous particulates and a volatile organic fraction ("VOF") comprising high molecular weight hydrocarbons resulting from unburned and partially burned diesel fuel and lubricating oil. The VOF is present in the diesel exhaust as either a vapor phase or a liquid phase, or both, depending on the temperature of the ex- haust. Generally, it is not feasible to attempt to remove or treat the dry, solid carbonaceous particulates compo¬ nent of the total particulates by catalytic treatment, and it is the VOF component which can be most effectively re- moved by conversion via utilization of an oxidation cata¬ lyst. Therefore, in order to reduce the total particu¬ lates discharged so as to meet present and impending Gov¬ ernment regulations concerning maximum allowable total particulates, the volatile organic fraction, or at least a portion thereof, must be oxidized to innocuous C0 2 and H 2 0 by being contacted with an oxidation catalyst under suit¬ able reaction conditions. The required U.S. Government limits for 1991 on HC, CO, nitrogen oxides ( "NO χ ") and total particulate emissions ("TPM") in diesel engine ex- haust have been largely met by suitable engine design mod¬ ifications. For 1994 the HC, CO and NO χ limits remain un¬ changed from 1991 standards but the upper limit on TPM will be reduced from the 1991 level of 0.25 grams per horsepower-hour ("g/HP-hr") to 0.10 g/HP-hr. Although the oxidation catalysts of the present invention, when em¬ ployed as a diesel exhaust treatment catalyst, are effec¬ tive for effectuating a reduction in total particulates, they are also capable, especially with the optional addi¬ tion of platinum or other catalytic metals as described below, of providing the added advantage of also oxidizing a portion of the HC and CO contained in the gaseous com¬ ponent of the diesel engine exhaust. When sulfur or sul¬ fur compounds are present in the exhaust in significant quantities, platinum is eliminated or used in limited amounts so as not to promote the unwanted effect of exces¬ sive oxidation of S0 2 to S0 3 . Further, the zeolite component of the present inven¬ tion is able to trap hydrocarbon molecules which might otherwise, during periods when the exhaust gas is rela- tively cool, escape untreated from the catalyst. It is believed that the trapped hydrocarbons are either oxidized within the zeolite or released from the zeolite only when the temperature of the catalyst composition is high enough to effectively catalyze oxidation of the trapped hydrocar¬ bons, or both. A basic and novel characteristic of the present in¬ vention is believed to reside in a catalyst composition comprising the defined combination of ceria, zeolite and, optionally, alumina, and one or both of the optional dop¬ ing of the zeolite and dispersal of the catalytic metals, platinum or palladium as part of the composition and in the use thereof to treat diesel exhaust streams. As noted above, the bulk ceria and the bulk alumina may each have a surface area of at least about 10 m 2 /g, for example, at least about 20 m 2 /g. Typically, the bulk alumina may have a surface area of from about 120 to 180 m"/g and the bulk ceria may have a surface area of from about 70 to 150 m : /g. The fact that a diesel oxidation catalyst composition which may contain activated alumina as a major component thereof has proven to be successful is in itself surpris¬ ing, in view of the consensus of the prior art that alumi- na, if used at all in diesel oxidation catalysts, must be a low surface area alumina (α-alumina) and/or be used in conjunction with sulfate-resistant refractory metal oxides such as zirconia, titania or silica. It has nonetheless been found that in accordance with one aspect of the pres- ent invention, surprisingly, a catalyst composition com¬ prising a combination of high surface area ceria, a suit¬ able zeolite, and, optionally, high surface area alumina, provides a catalytic material which effectively catalyzes the oxidation of the volatile organic fraction so as to provide a significant reduction in total particulates in diesel engine exhaust and is capable of adsorbing and cat¬ alyzing the combustion of gaseous hydrocarbons. It should be noted that the prior art generally considers refractory base metal oxides used in diesel oxidation catalysts to be merely supports for the dispersal thereon of catalytically active metals such as platinum group metals. In contrast, the present invention teaches that a catalytic material comprising ceria and, optionally, alumina of sufficiently high surface area (10 m 2 /g or higher) and zeolite, dis¬ persed on a suitable carrier, provides a durable and ef¬ fective diesel oxidation catalyst. It has further been found that beneficial effects are attained by the optional incorporation of platinum or pal¬ ladium in the catalyst composition, provided that in the case of platinum, the platinum is present at loadings much lower than those conventionally used in oxidation cata¬ lysts. If the catalytic metal platinum or palladium is added to the catalytic composition, it serves to catalyze the oxidation of gas-phase HC and CO pollutants as an added benefit. However, such catalytic metal is not needed to supplement the action of the ceria-zeolite or ceria-alumina-zeolite catalytic material in reducing total particulate emissions. Neither the platinum or palladium catalytic metal nor the metals or hydrogen used to dope the zeolite appear to significantly affect the rate of particulates conversion. The Zeolite The zeolite employed serves both to catalyze the ox¬ idation of VOF and to crack the larger VOF molecules and, during periods of relatively low temperature operation, to trap gas-phase hydrocarbons within the zeolite pores. If the zeolite has been doped with one or more catalytic met¬ als or hydrogen, the trapped gas-phase hydrocarbons are brought into intimate contact with the catalytically ac¬ tive cations which facilitates oxidation of the hydrocar¬ bons. In any case, the zeolite pores also serve to retain some of the gas-phase hydrocarbons during start-up or oth¬ er periods when the catalyst is relatively cool and there¬ fore less effective in catalyzing oxidation reactions, and to release the hydrocarbons only when the catalyst has been heated to higher temperatures. The higher tempera- tures impart sufficient energy to the trapped hydrocarbon molecules to enable them to escape the zeolite pores, but also enhance oxidation of the hydrocarbons in contact with the catalyst. The zeolite therefore serves not only as a catalyst for VOF oxidation, but as a hydrocarbon filter which traps hydrocarbons during periods of relatively low temperature and concomitant low catalytic activity and re¬ tains them until they can be efficiently oxidized by the catalyst during periods of relatively high temperature. The Carrier (Substrate) The carrier used in this invention should be rela¬ tively inert with respect to the catalytic composition dispersed thereon. The preferred carriers are comprised of ceramic-like materials such as cordierite, α-alumina, silicon nitride, zirconia, mullite, spodumene, alumina- silica-magnesia or zirconium silicate, or of refractory metals such as stainless steel. The carriers are prefer- ably of the type sometimes referred to as honeycomb or monolithic carriers, comprising a unitary body, usually cylindrical in configuration, having a plurality of fine, substantially parallel gas flow passages extending there¬ through and connecting both end-faces of the carrier to provide a "flow-through" type of carrier. Such monolithic carriers may contain up to about 700 or more flow channels ("cells") per square inch of cross section, although far fewer may be used. For example, the carrier may have from about 7 to 600, more usually from about 200 to 400, cells per square inch ("cpsi"). While this discussion and the following examples re¬ late to flow- hrough type carrier substrates, wall-flow carriers (filters) may also be used. Wall-flow carriers are generally similar in structure to flow-through carri- ers, with the distinction that each channel is blocked at one end of the carrier body, with alternate channels blocked at opposite end-faces. Wall-flow carrier sub¬ strates and the support coatings deposited thereon are necessarily porous, as the exhaust must pass through the walls of the carrier in order to exit the carrier struc¬ ture. The Catalytic Material The ceria-zeolite or ceria-alumina-zeolite catalytic material may be prepared in the form of an aqueous slurry of ceria, alumina (optional) and zeolite particles, the particles optionally being impregnated with the platinum or palladium catalytic metal component if one is to be dispersed on the catalytic material, and the zeolite may optionally be doped, e.g., may be ion-exchanged with sel¬ ected cations. The slurry is then applied to the carrier, dried and calcined to form a catalytic material coating ( "washcoat" ) thereon. Typically, the ceria, optional alu¬ mina and zeolite particles are mixed with water and an acidifier such as acetic acid, nitric acid or sulfuric acid, and ball milled to a desired particle size. Dispersed Catalytic Metals: The optional platinum or palladium catalytic metal component is, when used, dis¬ persed onto the ceria and zeolite particles or onto the ceria, alumina and zeolite particles. In such case, the ceria, zeolite and optional alumina act not only as cata- lysts in their own right but also as a support for the op¬ tional platinum or palladium catalytic metal component. (Such platinum or palladium dispersed onto the zeolite is not to be confused with the optional metals - which may be, among others, platinum and/or palladium - with which the zeolite may be doped by ion-exchange or otherwise as discussed in more detail below). The dispersal of the platinum and/or palladium onto the ceria, the optional alumina and the zeolite may be carried out after the ceria-zeolite or ceria-alumina-zeo- lite catalytic material is coated as a washcoat onto a suitable carrier, by impregnating the coated carrier with a solution of a suitable platinum and/or palladium com¬ pound, followed by drying and calcination. However, the ceria particles or both the ceria and alumina particles may be impregnated with a suitable platinum compound be¬ fore the ceria or ceria-alumina material (plus the zeo¬ lite) is applied to the carrier. The latter technique reduces the amount of platinum and/or palladium dispersed onto the outer surface of the zeolite particles, thereby increasing the amount of catalytic material dispersed on the high surface ceria or ceria-alumina relative to the zeolite. In either case, the optional dispersed platinum or palladium metal may be added to the ceria-zeolite or ceria-alumina-zeolite catalytic material as, e.g., a so¬ lution of a soluble platinum compound, the solution serv¬ ing to impregnate the ceria and optional alumina particles (or a coating thereof on the carrier) and the zeolite, if present at this point of the process. The impregnated components may then be dried and the platinum or palladium fixed thereon. Fixing may be carried out by calcination or by treatment with hydrogen sulfide or by other known means, to render the metal in water-insoluble form. Suit- able platinum compounds for use in the foregoing process include potassium platinum chloride, ammonium platinum thiocyanate, amine-solubilized platinum hydroxide and chloroplatinic acid, as is well-known in the art. During calcination, or at least during the initial phase of use of the catalyst, such compounds, if present, are converted into the catalytically active elemental platinum metal or its oxide. Palladium nitrate or palladium analogs of the aforementioned platinum compounds may be used to provide palladium. Doped Metals or Hydrogen: The zeolite used may op¬ tionally be doped either by conventional acid treatment to convert the zeolite to the acid form or by conventional ion-exchange techniques to exchange catalytically active metal cations for cations of the zeolite, or by any other suitable technique which disposes the catalytic metal or metals or hydrogen within the pores of the zeolite. (Con¬ ventional acid treatment of a zeolite to convert it to an acid [hydrogen] form of the zeolite is, for economy of ex¬ pression, referred to herein as doping the zeolite with hydrogen.) For doping the zeolite with metals, conven¬ tional ion-exchange techniques including repeated flushing with a solution of suitable metal compounds may be carried out. However, other doping techniques may be employed. For example, if the zeolite pores are flooded with a solu¬ tion of, e.g., tetraammine platinum hydroxide or tetraam- mine platinum chloride, the platinum dissociates as a cat¬ ion, and this positively charged platinum ionic species will adhere to negatively charged sites within the zeolite pores, even if the conventional ion-exchange technique of repeated exchanges of the solution with the zeolite is not carried out. Other techniques are not excluded, as long as the metals are dispersed throughout or within the zeo- lite pores. The conventional practice of indicating the catalytic moiety used to dope a zeolite by using it as a prefix for the zeolite is followed herein and in the claims. Thus, "H-Mordenite" indicates the hydrogen form of Mordenite, i.e., a hydrogen-doped Mordenite, "Fe-Beta" indicates an iron-doped Beta zeolite, "Fe-Pt-Beta" indi¬ cates an iron and platinum-doped Beta zeolite, etc. Preparing The Catalyst Composition Generally, a slurry of ceria particles, activated alumina particles and zeolite particles will be deposited upon the carrier substrate and dried and calcined to ad¬ here the catalytic material to the carrier and, when the dispersed platinum or palladium compound is present, to revert the platinum or palladium compound to the elemental metal or its oxide. The zeolite particles may optionally have been doped with one or more metals, e.g., with a com¬ bination of platinum and/or base metals, or acid-treated to dope the zeolite with hydrogen, i.e., to provide an acid form of the zeolite. When the catalytic material or any component is ap¬ plied to a suitable honeycomb carrier, such as described above, the amount of the component is conventionally ex¬ pressed herein and in the claims as weight of component per unit volume of catalyst, as this measure accommodates the presence of different sizes of catalyst composition voids provided by different carrier wall thicknesses, gas flow passage dimensions, etc. Grams per cubic inch ("g/in 3 ") units are used herein and in the claims to ex- press the quantity of relatively plentiful components such as the ceria-alumina-zeolite catalytic material, and grams per cubic foot ("g/ft 3 ") units are used to express the quantity of the sparingly-used ingredients, such as the platinum metal. For typical diesel exhaust applications, the ceria-zeolite or ceria-alumina-zeolite catalytic ma¬ terial of the present invention generally may comprise from about 0.25 to about 4.0 g/in 3 , preferably from about 0.25 to about 3.0 g/in 3 of the coated carrier substrate. The catalytic material may optionally also include from about 0.1 to 60, preferably from about 0.1 to 15 g/ft 3 of dispersed platinum or from about 0.1 to 200, preferably from about 20 to 120 g/ft 3 of dispersed palladium. The zeolite may optionally be doped with from about 0.1 to 50 g/ft 3 of metal, e.g., from about 1 to 60 g/ft 3 of precious metal, and/or from about 0.1 to 200 g/ft 3 of base metal or the zeolite may be converted to its hydrogen form. The conversion to hydrogen form may be a 5 to 100 percent hy¬ drogen exchange. Without wishing to be bound by a particular theory, applicants offer the following hypothesis to explain the superior performance, when used to treat diesel engine ex¬ haust, of the ceria-zeolite or ceria-alumina-zeolite cat¬ alytic materials according to this invention. It is be- lieved that diesel exhaust contains a significant propor¬ tion of gases or vapors which are close to their dew point, i.e., close to condensing to a liquid, and thereby adding to the VOF portion of the particulates at the con¬ ditions obtaining in the exhaust pipe. These "potential particulates" condense in the ceria-zeolite or ceria-alu¬ mina-zeolite catalytic materials, their condensation being enhanced by a capillary condensation effect, a known phe¬ nomenon in which a capillary-like action facilitates con¬ densation of oil vapors to liquid phase. The small pore size of the high surface area ceria or ceria and alumina components of the catalytic material is believed to pro¬ vide such capillary condensation action for the VOF. Gen¬ erally, the higher the surface area of the ceria and op- tional alumina, the smaller is the pore size. As the ex¬ haust temperature increases during increased work loads imposed on the diesel engine, the condensed hydrocarbon liquids (condensed VOF) are desorbed from the ceria or ceria-alumina components of the catalytic material and. volatilize, at which time the catalytic effect of the ce¬ ria or the ceria-alumina components of the catalytic ma¬ terial, which provide numerous active sites, and the cracking effect attained by contact of the VOF with the surface of the zeolite, enhances gas-phase oxidation, i.e., combustion, and cracking of the desorbed, re-vola¬ tilized hydrocarbon (VOF) vapors. Even if a proportion of the vapors re-volatilized from the condensate is not com¬ busted, the cracking of heavy VOF components to lighter hydrocarbons on the outer surface of the zeolite reduces the total amount of condensibles, so that the total par¬ ticulates output from the diesel engine is concomitantly further reduced. In this latter regard, the ceria or ce¬ ria-alumina component of the catalytic material is be- lieved to act as a trap and a storage medium for condensed or condensible VOF during relatively cool phases of the exhaust, and volatilizes the VOF and cracks it during rel¬ atively hot phases. The zeolite serves, during relatively cool phases of the exhaust, to trap gas-phase hydrocarbon components and retain them until temperatures high enough for the platinum or other metal to oxidize at least some of the gas-phase HC are attained. The porous nature of the ceria-zeolite or ceria-alumina-zeolite catalytic ma¬ terial is also believed to promote rapid diffusion of the VOF throughout the washcoat structure, thereby facilitat¬ ing relatively low temperature gasification and oxidation of the VOF upon increases in temperature of the catalyst during higher engine load (increased exhaust gas tempera¬ ture) cycles. The presence of sulfates does not signifi- cantly adversely affect the capacity of the ceria-zeolite or ceria-alumina-zeolite catalytic material to reduce par¬ ticulate emissions. Generally, other ingredients may be added to the cat- alyst composition of the present invention such as conven¬ tional thermal stabilizers for the alumina, e.g., rare earth metal oxides such as ceria. Thermal stabilization of high surface area ceria and alumina to militate against phase conversion of these oxides to less catalytically.ef¬ fective low surface area forms is well-known in the art, although thermal stabilization of alumina is not usually needed for diesel exhaust service wherein exhaust temper¬ atures are typically lower than for gasoline-fueled en- gines. Such thermal stabilizers may be incorporated into the bulk ceria or into the optional bulk activated alu¬ mina, by impregnating the ceria (or alumina) particles with, e.g., a solution of a soluble compound of the stabi¬ lizer metal, for example, an aluminum nitrate solution in the case of stabilizing bulk ceria. Such impregnation is then followed by drying and calcining the impregnated ce¬ ria particles to convert the aluminum nitrate impregnated therein into alumina, to thermally stabilize the ceria. A suitable technique is shown in U.S. Patent 4,714,694 to C.Z. Wan et al (the disclosure of which is incorporated by reference herein), in which ceria particles are impreg¬ nated with a liquid dispersion of an aluminum compound, e.g., an aqueous solution of a soluble aluminum compound such as aluminum nitrate, aluminum chloride, aluminum oxy- chloride, alumnium acetate, etc. After drying and calcin¬ ing the impregnated ceria in air at a temperature of, e.g., from about 300°C to 600°C for a period of 1/2 to 2 hours, the aluminum compound impregnated into the ceria particles is converted into an effective thermal stabil- izer for the ceria, to provide an "alumina-stabilized ce¬ ria". In addition, the catalyst compositions of the inven¬ tion may contain other catalytic ingredients such as other base metal promoters or the like. However, in one embodi- ment, the catalyst composition of the present invention consists essentially only of the high surface area ceria and the zeolite, and, optionally, the high surface area alumina, preferably present in the weight proportions giv- en above, with or without thermal stabilizers impregnated into the alumina and ceria and, optionally, including pal¬ ladium or limited amounts of platinum dispersed thereon, and optionally, employing a doped zeolite as described above. Examples A and B show typical methods of preparing metal-doped zeolites which are useable as components of the catalytic material of the present invention. Example A Preparation of Fe-Beta Zeolite To prepare a sample of iron-exchanged Beta zeolite (Fe-Beta zeolite), 17 grams of iron [II] sulfate was dis¬ solved in 800 ml water. One hundred grams of Beta zeolite was added to the solution which was then stirred for 1 hour at a temperature of 70°C to 80°C. The resulting slurry was then filtered and washed with 2 liters of wa¬ ter, dried at 120°C and calcined at 540°C. The resulting material comprised 1.65% by weight iron. This technique was employed to prepare the Fe-Beta zeolite catalysts used in the following Examples. Example B Preparation of Pt-Fe-Beta Zeolite A solution was prepared using 0.54 grams of tetraam- mine platinum chloride in 500 ml water, to which 100 grams of Beta zeolite was added. The mixture was stirred for 24 hours at room temperature so that platinum ions replaced sodium ions in the zeolite material. The slurry was then filtered and washed with 2 liters water, dried and cal¬ cined at 540°C. The resulting calcined, platinum ion-ex¬ changed Beta zeolite was then ion-exchanged with iron [II] sulfate by adding the zeolite to a solution of iron [II] sulfate equivalent to 17 grams of iron [II] sulfate in 800 ml water. The solution was allowed to stand for about 1 hour and was then stirred at 70°C to 80°C for 1 hour. The resulting slurry was then filtered, washed with water, dried at 120°C and calcined at 540°C. On a dry basis, the Beta zeolite comprised 1.6% by weight iron and 0.25% by weight platinum. This technique was employed to prepare the Fe-Pt-Beta zeolite catalyst components used in the following Examples. Example 1 A. A catalyst according to the present invention was prepared by coating a honeycomb monolith with a catalytic material comprised of a zeolite, bulk ceria and bulk alu- mina to provide 0.84 g/in 3 γ-alumina, 0.83 g/in 3 alumina- stabilized ceria and 0.83 g/in 3 Fe-Beta zeolite. The hon¬ eycomb monolith was a cordierite substrate measuring 9 inches in diameter by 6 inches long and having 400 cpsi. The catalyst material also provided 2.5 g/ft 3 of platinum, 80 percent by weight of which was dispersed on the alumi¬ na, and 20 percent by weight of which was dispersed on the ceria. This catalyst was designated E-l. B. For comparison, three other catalysts were pre- pared to provide a series of three otherwise identical compositions containing a ceria-alumina catalytic material but no zeolite and, in two cases, having platinum dis¬ persed thereon. The platinum loadings of these compara¬ tive catalysts were 0.0, 0.5 and 2.0 g/ft 3 platinum. Each comparative catalyst comprised a γ-alumina undercoat at a loading of 1.0 g/in 3 upon which was coated a top coat lay¬ er comprised of 1.05 g/in 3 γ-alumina plus 0.90 g/in 3 alu¬ mina-stabilized ceria. The alumina-stabilized ceria con¬ tained 2.5 weight percent A1 2 0 3 based on the combined weight of bulk ceria and stabilizing alumina dispersed therein. The catalysts were coated onto a 9 inches dia¬ meter by 6 inches long, 400 cpsi cordierite substrate. As used herein, "cpsi" is an abbreviation for cells per square inch, denoting the number of gas flow passages per square inch of face area of the substrate. The resulting catalyst samples were designated as C-l (0.0 g/ft 3 plati¬ num, aged 24 hours as described below), C-2 (0.5 g/ft 3 platinum, aged 25 hours) and C-3 (2.0 g/ft 3 platinum, aged 24 hours ) . C. The four catalyst samples were conditioned prior to evaluation using an aging cycle involving 20 minutes each at Modes 2,6 and 8 of the European Thirteen Mode Cy¬ cle Test Procedure (ECE R.49 Thirteen Mode Cycle). This Test Procedure is set forth in the Society of Automotive Engineers Publication, SAE Paper 0880715, published at the International Congress and Exposition, Detroit, Michigan, February 29 through March 4, 1988, by Georgio M. Cornetti et al. The disclosure of this SAE publication is incorpo¬ rated by reference herein. Prior to testing to develop the data of TABLE I-B and Figures 1-5, the three catalyst samples were aged 24 or 25 hours as indicated above on a Cummins 6BT turbocharged diesel engine having a 5.9 liter displacement and rated at 230 horsepower. For both aging and test purposes, the engine was run with low sulfur fuel (0.05 weight percent sulfur) under steady state conditions using test modes selected from the aforesaid European Thirteen Mode Cycle Test Procedure. The engine conditions for the test modes along with average (for five runs) catalyst inlet temperatures and baseline emissions (of untreated engine exhaust) are shown in TABLE I-A. TABLE I-A Cummins 6BT 230 HP Turbocharged Diesel Engine, 5.9 Liter Displacement, Steady State Conditions For Catalyst Tests Engine Conditions Average TABLE I-A(Cont. ) Cummins 6BT 230 HP Turbocharged Diesel Engine, 5.9 Liter Displacement, Stead State Conditions For Catal st Tests grams per brake horsepower hour The conversion activities of each of these catalysts was evaluated in selected steady state modes of an ECE R.49 Thirteen Mode Cycle Test Procedure using a 1991 Cummins 6BT (6 liter DI/TC) engine after approximately 24 hours aging, as described above. The exhaust inlet tem- perature and the conversion rate for the conversion of VOF, TPM (total particulates), hydrocarbons and carbon monoxide were measured for various modes of the test, and are set forth in the following TABLE I-B. TABLE I-B(Cont. ) ECE Thirteen Mode Cycle Test The data of TABLE I-B are shown graphically in Fig¬ ures 1-5 and show that the catalyst according to the pres¬ ent invention (E-l) exhibited high levels of gas-phase hy¬ drocarbon removal in the lowest temperature modes (modes 1 and 2) of the test (72% and 61%) compared to the compara¬ tive samples under the same conditions (e.g., 37% and 39% for catalyst C-3). At higher temperatures, e.g., 340° and above, gas-phase hydrocarbon conversion rates of sample E-l were comparable to sample C-3 but still markedly supe¬ rior to samples C-l and C-2. The superior gas-phase hy¬ drocarbon conversion performance of sample E-l, especially at low temperature regimes, is attributed to the Fe-Beta zeolite. Carbon monoxide conversion was essentially the same for sample E-l and the comparative sample C-3, as will be readily appreciated from Figure 2, indicating that the addition of the iron-Beta zeolite did not appear to have an effect on carbon monoxide conversion efficiency. The overall VOF removal performance of sample E-l was slightly better than any of the comparative samples. Fig- ure 3 graphically illustrates VOF conversion efficiency of catalyst E-l and comparative sample C-3. Example 2 Catalyst Comprising Fe-Beta Zeolite and Platinum on Alumina and Ceria A.(l) A catalyst component comprising platinum dis¬ persed on Fe-Beta zeolite and on ceria was prepared as follows. Two hundred grams of Fe-Beta zeolite containing 1.65% by weight iron was made into a water-based slurry and milled for 3 hours. The pH was adjusted to 4.5 by the addition of monoethanolamine. An amine-solubilized plati¬ num hydroxide solution was added to the slurry, followed by acetic acid to precipitate the platinum, which consti¬ tuted 0.029% by weight. The slurry was further milled to reduce the particle size so that 90 percent of the parti¬ cles have a diameter of 12 microns or less. (2) Separately, a similar platinum salt solution was slowly added to 650 grams of ceria having a BET sur¬ face area of 143 m 2 /g that had previously been impregnated with 2.5% by weight alumina stabilizer (basis, weight of ceria plus alumina stabilizer) by a conventional technique of impregnating the ceria with a solution of an alumina precursor and drying and calcining the impregnated ceria. Sufficient platinum salt solution was added to deposit 0.029% platinum onto the ceria, by weight of ceria plus platinum. The platinum was fixed onto the ceria by adding 5 cc of acetic acid per 100 grams of solids to the Pt/ce- ria mixture. The Pt/ceria mixture was milled in water so that 90% of the particles have a diameter of 12 microns or less to produce a slurry having 47.8% solids. (3) Portions of the.two slurries of parts (1) and (2) were mechanically mixed to produce a slurry comprising about 50% ceria, 50% Fe-Beta zeolite and 0.029% platinum, all by weight (dry basis). A portion of the mixed slurry was dried overnight at 100°C in air and then calcined in air at 450°C for 1 hour. A portion of the dried, calcined powder was made into a slurry and coated onto a cordierite honeycomb 1.5 inches in diameter, 3 inches long and haying 400 cpsi, at a loading of 1.0 g/in 3 . This washcoat load¬ ing yielded 0.5 g/ft 3 of platinum. The coated monolith was dried and calcined and designated E-2. B.(l) A catalyst was prepared from a slurry compris¬ ing three types of particles, platinum-bearing ceria par¬ ticles, platinum-bearing alumina particles and Fe-Beta zeolite particles, i.e.. Beta zeolite ion-exchanged with iron. These particles were prepared as separate slurries before being mixed together. (2) The slurry of platinum-bearing ceria particles was prepared by impregnating ceria particles having a BET surface area of 143 m /g with a platinum salt solution as described above in part A(l) to obtain a platinum metal loading of 0.035% upon calcination. The resulting parti¬ cles were milled to an average particle size of less than 12 microns. (3) The slurry of platinum-bearing alumina parti¬ cles was prepared by impregnating alumina having a BET surface area of 150 m 2 /g with a platinum salt solution to yield 0.138% by weight platinum metal upon calcination. The particles were milled to an average particle size less than 12 microns. (4) A slurry of Fe-Beta zeolite particles was pre- pared by milling the particles to an average particle size of less than 12 microns. (5) The foregoing three slurries were mechanically mixed to form a composite slurry yielding 33.18% ceria, 33.18% Fe-Beta zeolite, 33.58% alumina and 0.058% platinum by weight (dry basis). The composite slurry was coated onto an oval-shaped honeycomb monolith having cross sec¬ tion dimensions of 3.18 X 6.68 inches, a length of 3.15 inches and 400 cpsi, at a total loading of 2.5 g/in 3 , consisting of 0.84 g/in 3 alumina, 0.83 g/in 3 ceria, 0.83 g/in 3 Fe-Beta zeolite and 0.0014 g/in 3 (equivalent to 2.4 g/ft 3 ) platinum. The catalyst-coated honeycomb carrier was dried in air at 100°C and then calcined at 450°C for 2 hours. A sample core 3 inches long and 1.5 inches in.di¬ ameter was drilled from the calcined honeycomb and desig¬ nated E-3. (6) Fe-Beta zeolite having 1.65 weight percent iron was made into an aqueous slurry and milled so that about 90 percent of the particles had a diameter of 12 mi¬ crons or less. The slurry was then coated onto a carrier monolith 1.5 inches in diameter and 3.0 inches in length having 400 cpsi. The coated carrier was calcined at 450°C in air for 1 hour and had a washcoat loading of 1.0 g/in 3 (dry basis). This sample was designated C-4. C. The catalysts designated E-2, E-3 and C-4 were tested in a laboratory diagnostic reactor for comparison. The inlet stream of the diagnostic reactor comprised 200 ppm heptane measured as C χ , 4.5% C0 2 , 50 ppm S0 2 , 1000 ppm NO, 200 ppm CO, 10% 0 2 , 10% H 2 0, balance N 2 . The inlet temperatures of the test gas stream to the catalyst were 275, 350, 425 and 500°C. The results of the diagnostic laboratory reactor tests as set forth in Figures 6-9 show that the catalysts of the present invention, E-2 and E-3, exhibited substan¬ tially better gas phase HC activity at a space velocity ("sv" ) of 50,000/hr and better HC activity at sv=90,000/hr than did the catalyst containing only Fe-Beta zeolite (C-4). Furthermore, E-2 gave better results than E-3 de¬ spite having a lower Pt loading (0.5 vs. 2.5 g/ft 3 ). It is also clear that the catalysts of the present invention gave much better CO conversion activity than did the cata¬ lyst containing only Fe-Beta zeolite, the latter exhibit- ing net negative CO conversion due to partial oxidation of HC to form CO in an amount to give a higher concentration in catalyst outlet stream than in the inlet stream. Fin¬ ally, the catalysts of the present invention exhibit very low S0 2 oxidation levels due to the low Pt loading, indi¬ cating control of sulfate-make by the catalyst which would otherwise contribute to particulate emissions. As used herein and throughout this application space velocity ("sv") has the usual meaning of the volumes of the exhaust stream or test gas, measured at standard conditions of temperature and pressure, passing through the volume of the catalyst composition (the dimensional volume of the coated honeycomb monolith) per hour. Example 3 Catalyst Comprising Pt/CeO^ , Pt/Al 2 0 3 and Fe-Beta Zeolite A. A catalyst according to the present invention was prepared by coating two differently configured honeycomb carriers with identical loadings of the same catalytic ma¬ terial. One honeycomb carrier to be tested on an Audi 100 automobile as described below measured 5.66 inches in dia¬ meter by 6 inches long and had 400 cpsi and was designated an "A-type carrier". A second honeycomb carrier to be tested on a Mercedes Benz 200D automobile as described be¬ low was of oval configuration and measured 3.03 inches by 5.78 inches (minor and major axes of the oval face) by 6 inches long and had 200 cpsi. This carrier was designated an "M-type carrier". Both the A-type carrier and the M- type carrier were coated with 2.5 g/in 3 of a washcoat com¬ prising 33% by weight alumina-stabilized ceria and 34% by weight γ-alumina with platinum dispersed on both the ceria and the alumina as described above in Example 2. The washcoat further comprised 33% by weight Fe-Beta zeolite having 1.65% iron by weight. The washcoat yielded a plat¬ inum loading of 2.5 g/ft 3 platinum distributed equally on the ceria and alumina; there was no platinum on the Fe- Beta zeolite. These catalysts were each designated E-4. B. A second pair of catalysts according to the pres¬ ent invention was prepared as described in part A above, i.e., one was prepared using an A-type carrier and one us¬ ing an M-type carrier, except that the platinum loading was 10 g/ft 3 dispersed equally on the ceria and alumina, with no platinum on the Fe-Beta zeolite. These catalysts were each designated E-5. C.(l) A comparative catalyst was prepared by coating an M-type carrier with 2.5 g/in 3 of a catalyst washcoat consisting of 46% by weight alumina-stabilized ceria hav¬ ing a BET surface area of 143 m 2 /g and 54% by weight γ-alumina having a BET surface area of 150 m 2 /g. The cat- alyst was prepared by separately impregnating bulk alumina and bulk ceria with an amine-solubilized platinum hydrox¬ ide solution, precipitating the platinum with acetic acid and milling the particles to a size of 90 percent of the particles having a diameter of 12 microns or less. The two slurries were then blended to provide a 50% solids slurry which was used to coat the honeycomb carrier. The washcoat yielded a platinum loading of 2.0 g/ft 3 equally dispersed on the ceria and alumina. This catalyst was de¬ signated comparative sample C-X. (2) Catalyst pairs respectively designated C-5 and C-6 were prepared as described for catalyst C-X, i.e., two C-5 samples, one on each of an A-type carrier and an M- type carrier and two C-6 samples, one on each of an A-type and an M-type carrier, except that the platinum loadings were 5.0 g/ft 3 on the C-5 samples and 10 g/ft 3 on the C-6 samples. (3) A commercial catalyst material comprised a washcoat of 85-90% by weight alumina, 5-7% by weight vana¬ dium oxide (V ; 0 5 ) and 1-2% by weight platinum coated onto a honeycomb carrier measuring 5.66 inches in diameter by 6 inches long and having 400 cpsi. The catalyst had 60 g/ft 3 of catalytic material. This catalyst was designated C-7. D. The catalysts of parts A, B and C were tested for hydrocarbon conversion, CO activity and S0 2 conversion us¬ ing the exhaust from an Audi 100 automobile having a 5 cylinder, 2.5 liter, direct injected/turbocharged engine with intercooling and exhaust gas recycle and a Mercedes Benz 200D automobile having a 4 cylinder, 2.0 liter, in¬ direct injected/naturally aspirated engine. The tests were conducted using a chassis dynamometer and emissions measurement instrumentation and techniques for total gas-phase hydrocarbons (HC), carbon monoxide (CO, nitrogen oxides NO χ ) and total particulates (TPM). The emissions evaluation test used was the European transient test known as Cycle "A". This test consists of two parts. The first, the ECE part, is characterized by lower loads and cooler exhaust temperatures than the sec¬ ond part, the EUDC (Extra Urban Driving Cycle). Emissions are measured for each of these parts of the test and the results are weighed and combined to give emissions for the overall Cycle "A" test. Emissions are expressed in grams/kilometer (g/km). The results of the Cycle A tests are set forth in TABLE II-A and TABLE II-B. TABLE II-A TABLE II-A(Cont. stances by catalytic treatment. TABLE II-B(Cont. ) Test Results for Mercedes Benz 200D Diesel Automobile Carbon Hydrocarbons Monoxide Particulates Test Cata- Pt Load. Part lyst g/ft 3 g/km 1 / % 2 q/km 1 / % 2 g/km 1 / % 2 Emissions in exhaust. Percent of emissions converted to innocuous sub¬ stances by catalytic treatment. From the data in the foregoing TABLES II-A and II-B it can be seen that samples E-4 and E-5 according to the present invention gave higher gas-phase hydrocarbon con¬ version (about 80%) in the ECE part of the test for the Audi 100 automobile than comparative catalysts C-5, C-6 having comparable platinum loadings but without Fe-Beta zeolite, and without significant gain in S0 2 oxidation. Further, catalysts E-4 and E-5 according to the present invention gave better hydrocarbon conversion performance than the commercial catalyst C-7, which has a much higher platinum loading. In the hotter EUDC part of the test, on the Audi 100 automobile, catalyst samples of the present invention (E-4, E-5) exhibited superior particulates removal com¬ pared to the comparative commercial catalyst C-7, as did the comparative catalysts C-5 and C-6. This is probably due to the high degree of S0 2 oxidation caused by the high platinum loading of the commercial catalyst, the resulting sulfate make adding to the mass of the particulates. Overall, the catalysts according to the present invention (E-4, E-5) gave better gas-phase hydrocarbon conversion and particulates removal than the comparative commercial catalyst (C-7) and the comparative catalysts (C-5, C-6). The overall performance of the catalysts for Cycle A was slightly better than the comparative samples. The results for the ECE portion of the tests on the Mercedes Benz automobile also show that catalysts accord¬ ing to the invention (E-4, E-5) had superior low tempera¬ ture gas-phase hydrocarbon conversion while maintaining acceptable particulates conversion. It is noted that all the catalysts except E-4 exhibited significant improvement in CO conversion, in comparison to the Audi 100 test. The improvement is attributed to the difference in exhaust temperature between the respective vehicles (on average 10°C higher for Mercedes than for Audi), highlighting the temperature sensitivity of CO conversion activity. The overall Cycle A results for the catalysts of the invention are substantially better than the comparative samples due to the better overall hydrocarbon conversion with compara¬ ble CO and particulates conversion. The better total par- ticulates conversion rates exhibited for the Audi automo¬ bile relative to the Mercedes automobile can be attributed to the lower VOF content of the Mercedes exhaust as com¬ pared to the Audi exhaust. Example 4 Examples of Ceria-Alumina With Pt-Beta Zeolite and Pt-Fe-Beta Zeolite A. Platinum ion-exchanged Beta zeolite was prepared by stirring 100 grams of Beta zeolite powder into a water solution containing 0.85 grams tetraammine platinum [II] chloride in 500 ml water. The resulting slurry was stir¬ red and allowed to stand for 24 hours, and then filtered, washed with 1 liter of water, dried overnight at 100°C and calcined at 540°C for 2 hours. The resulting Pt-Beta zeo¬ lite material contained 0.48% platinum by weight. A catalyst slurry was prepared comprising 33% by weight (dry basis) of the Pt-Beta zeolite and 67% (dry basis) of a mixture of equal parts by weight of ceria hav¬ ing a surface area of 143 m 2 /g and γ-alumina having a sur¬ face area of 150 m 2 /g. The slurry was coated onto a hon¬ eycomb carrier to provide a loading of 2 g/in 3 using 10% by weight additional γ-alumina as a binder. The coated honeycomb was exposed to a gaseous mixture of 10% steam in air at 400°C for 4 hours. This catalyst contained 10g/ft 3 Pt and was designated E-6. B. Beta zeolite was ion-exchanged with iron and then with platinum as generally described in Example B to yield a Beta zeolite comprising 1.65% by weight iron and 0.5% by weight platinum. The resulting Pt-Fe-Beta zeolite material was made into a slurry comprising 33% by weight (dry basis) Pt-Fe-Beta zeolite and 67% (dry basis) of a mixture of equal parts by weight of ceria having a surface area of 143 2 /g and γ-alumina having a surface area of 150 m 2 /g. The slurry was coated onto a honeycomb carrier at a loading of 2 g/in 3 using additional alumina as a binder. This catalyst contained 10 g/ft 3 Pt and was de¬ signated E-7. C. Catalysts E-6 and E-7 were tested in a diagnos¬ tic reactor through which a test stream identical to that described in part C. of Example 2 was passed. The hep¬ tane, carbon monoxide and S0 2 conversion rates were noted at the inlet temperatures of the test stream set forth be¬ low in TABLE III, in which the results are tabulated. TABLE III Conversion % Heptane at °C % CO at °C % S0 2 at °C Cat. 150 200 275 350 150 200 275 350 150 200 275 350 E-6 3.3 3.5 13 93 1.5 7 97 100 2.6 0 25 52 E-7 4.0 2.0 8.6 88 0 43 100 100 9.0 2 18 53 The data of TABLE III show both catalysts E-6 and E-7 were effective for the conversion of heptane and carbon monoxide, and that sample E-7 exhibits superior carbon monoxide conversion at 200°C and above relative to sample E-6, while providing comparable performance with respect to heptane conversion and S0 2 oxidation. Lube Oil Combustion Test and Test Results In many diesel engines the VOF in the diesel exhaust consists mainly of diesel lube oil which has been swept from the cylinder walls and comes through valve guides and turbocharger seals. A laboratory test was used to evalu¬ ate the relative performance of the catalyst powders of Example 5 below for burning diesel lube oil, as a model of the ability of the corresponding catalyst to catalyze the oxidation of the VOF in diesel engine exhaust. This test allows for the relative ranking of catalyst materials for their effectiveness of burning lube oil via the interac¬ tions between the catalyst and lube oil as they are heated together in air. Thus, a catalyst powder sample is mixed uniformly with a measured amount of lube oil. The mixture of cata¬ lyst and lube oil (about 10-30 mg) is placed into the quartz sample pan of a simultaneous TGA/DTA instrument (Thermal Sciences STA 1500) and heated in flowing air using a standard heating ramp (20°C/min.) from ambient temperature to 1000°C. The collected data, cumulative weight loss (TGA) and heat evolution (DTA) as a function of temperature, are normalized for the weight of catalyst sample and the amount of lube oil present. The total weight loss (TGA) measured is made up of water loss which occurs at about 100°C or less and lube oil loss either by volatilization or by combustion. The water loss occurs in a discrete step and can be thus differentiated from the lube oil losses. The exotherm (DTA Peak) is a measure of the lube oil loss due to combustion of the lube oil. These data are used to calculate a DTA Peak Area (uv-sec/mg-catalyst sample/mg-lube oil) which is used as the key measure of the catalyst's ability to catalyze the combustion of VOF (lube oil) in this test. Tests have been conducted in which catalyst powders were used to catalyze the combus- tion of lubricating oil and the same catalytic powders, were used to prepare catalysts supported on a substrate. Tests of such catalysts on diesel engine exhausts showed good correlation between the performance of the catalyst powder in the laboratory TGA and DTA tests and the per- formance of the corresponding catalyst in the engine test. Example 5 TGA/DTA Lube Oil Combustion Test FCC Cat (Ceria/Zeolite) A. A catalyst material according to the present in¬ vention designated E-8 comprised a mixture of 50% by weight bulk ceria having a BET surface area of 143 m /g and 50% by weight ZSM-5 H-zeolite by weight of the mix- ture. The catalyst was prepared by mixing the ceria and ZSM-5 powders, drying overnight at 100°C and calcining at 450°C for three hours. The catalyst material was wetted with a quantity of Cummins SAE 15 W/40 Premium blue diesel engine lube oil equal to 4% of the weight of the zeolite, and was then placed in a TGA/DTA analyzer. The results are set forth in the attached Figure 10. B. A series of seven other catalyst materials was prepared as generally described in Part A above, and were designated E-9 through E-15, respectively. The first three of these, E-9, E-10 and E-ll, comprised mixtures of ceria having a surface area of 143 m 2 /g on which platinum had been dispersed by the incipient wetness method to pro¬ vide 0.5 percent platinum thereon by weight of ceria plus platinum (Pt/Ceria) with H-Mordenite, Fe-Beta zeolite and H-ZSM-5, respectively. In each case, equal weights of Pt/ceria and zeolite were used. Samples E-12, E-13, E-14 and E-15 all comprised mixtures of zeolites, ceria and alumina, in which mixtures of equal weights of ceria plus alumina comprised 67% of the weight of the catalytic ma¬ terial, the zeolite (including doped metals) accounting for 33% by weight of the catalytic material. In these samples, the zeolites were doped by ion-exchange with .0.6 percent platinum by weight of the zeolite. The zeolites in these examples were Pt-ZSM-5, Pt-Y-zeolite, Pt-Fe-Beta zeolite and Pt-H-Beta zeolite, respectively. The ability of each of these sample materials to cat- alyze the combustion of the lube oil was tested using thermogravimetric analysis and differential temperature analysis (TGA/DTA) techniques. In TGA/DTA studies, the heat absorbed or released and the weight lost by a sample as a function of temperature are recorded simultaneously and can be used to evaluate the capability of the material to catalyze the combustion of the lube oil. Since, as discussed above, lube oil constitutes a significant por¬ tion of the VOF particulates in diesel engine exhaust, the results of the TGA/DTA study of lube oil-doped material reflects the ability of the material to combust the VOF particulates. The amount of lube oil added to the mate¬ rial (10% by weight) is far in excess of the amount of VOF hydrocarbons that a diesel catalyst would ordinarily be exposed to in the time frame of this test, but this quan- tity is used so that reproducible results may be obtained from this test. Each oil-wetted sample was placed in a quartz pan sample holder located inside an STA/1500 Simul¬ taneous Thermoanalyzer and heated in air at a rate of 20°C/min. from room temperature to 1000°C. The sample weights varied between 30 and 10 milligrams. For compari¬ son, a like quantity of lube oil was placed in the analy¬ sis chamber so that the TGA/DTA curves could be corrected for water loss and vaporization. In Figure 10, the ordinate on the left-hand side of the Figure relates to the lower curve and shows changes in the weight of the sample of oil-wetted material relative to temperature. The ordinate on the right-hand side of the Figure relates to the upper curve and shows the heat released from the sample per milligram of sample (catalyst plus lube oil) at various temperatures. The quantity of heat is measured by a thermocouple and is expressed in mi¬ crovolts per milligram of catalyst plus lube oil. The distinct rise in heat release at about 200°C indicates, the commencement of catalytic oxidation of the wetted oil, as does the loss in weight of the sample. One indication of the catalytic activity of the test material is the area under the DTA plot, referred to in TABLE IV as the DTA area. The DTA area is obtained by integrating the area under the curve and dividing by the lube oil weight loss as measured by TGA. The units are thus measured in micro¬ volt-seconds (generated by a thermocouple in response to the temperature change) per milligram of catalyst sample per milligram of lube oil. The units are abbreviated as "uv-s/mg/mg" in TABLE IV. Generally, the more catalytic¬ ally active the material, the greater is the DTA area: that is, the greater is the heat release generated by com¬ bustion of the lube oil as compared to the total amount of normalized lube oil loss (resulting from combustion plus volatilization). The following TABLE IV shows the DTA area for all the samples tested in this example. TABLE IV Sample (Percents By Weight) DTA Area uv-s/mg/mg E-8 50% CeO, plus 50% ZSM-5) 5,856 E-9 50% Pt/CeO, plus 50% H-Mordenite) 10,100 E-10 50% Pt/Ce0 2 plus 50% Fe-Beta Z) 14,800 E-ll 50% Pt/Ce0 2 plus 50% H-ZSM-5) 16,700 E-12 67% [Al.O./Ce0 2 ]* plus 33% Pt-ZSM-5) 12,975 E-13 67% [Al 2 0 3 /Ce0 2 ]* plus 33% Pt-Y-Z) 6,990 E-14 67% [Al.O,/Ce0 2 ]* plus 33% Pt-Fe-Beta Z) 10,200 E-15 67% [Al 2 0 3 /Ce0 2 ]* plus 33% Pt-H-Beta Z) 10,630 * 50% A1 2 0, plus 50% Ce0 2 The data of TABLE IV show that a variety of zeolites can be combined with ceria or ceria and alumina to yield materials that are catalytically active for the combustion of lube oil. Such catalytic activity, as discussed above, is indicative of the ability to reduce the VOF content of diesel exhaust. While the invention has been described in detail with respect to specific preferred embodiments thereof it will be appreciated that variations thereto may be made which nonetheless lie within the scope of the invention and the appended claims.

Claims: THE CLAIMS What is claimed is: 1. A catalyst composition for treating a diesel eng¬ ine exhaust stream containing a volatile organic fraction comprises a refractory carrier on which is disposed a coating of a catalytic material comprising a catalytically effective amount of ceria having a BET surface area of at least about 10 2 /g and a catalytically effective amount of a zeolite. 2. The catalyst composition of claim 1 further in¬ cluding a catalytically effective amount of alumina having a BET surface area of at least about 10 m 2 /g. 3. The catalyst composition of claim 1 wherein the zeolite comprises a three-dimensional zeolite character¬ ized by pore openings whose smallest cross-sectional di¬ mension is at least about five Angstroms and having a sil¬ icon to aluminum atomic ratio of greater than 5. 4. The catalyst composition of claim 1 wherein the zeolite comprises Beta zeolite. 5. The catalyst composition of claim 1 wherein the zeolite is selected from the group consisting of Y-zeo¬ lite, pentasil, Mordenite, and mixtures thereof. 6. The catalyst composition of claim 3, claim 4 or claim 5 further including a catalytically effective amount of alumina having a BET surface area of at least about 10 m 2 /g. 7. The catalyst composition of claim 2, claim 3 or claim 4 wherein the zeolite comprises from about 10 to 90 percent by weight, the alumina comprises from about 60 to 5 percent by weight, and the ceria comprises from about 60 to 5 percent by weight, of the combined weight of the .zeo¬ lite, the alumina and the ceria. 8. The catalyst composition of claim 2, claim 3 or claim 4 wherein the zeolite comprises from about 20 to 70 percent by weight, the alumina comprises from about 50 to 20 percent by weight, and the ceria comprises from about 50 to 20 percent by weight, of the combined weight of the zeolite, the alumina and the ceria. 9. The catalyst composition of claim 1, claim 2, claim 3 or claim 4 wherein the zeolite is doped with a catalytic moiety selected from the group consisting of one or more of hydrogen, platinum, rhodium, palladium, ruthe¬ nium, osmium, iridium, copper, iron, nickel, chromium and vanadium. 10. The catalyst composition of claim 9 wherein the zeolite is doped with the catalytic moiety by ion-exchang¬ ing the zeolite with cationic catalytic moiety. 11. The catalyst composition of claim 9 wherein the catalytic moiety comprises one or both of platinum and iron. 12. The catalyst composition of claim 11 wherein the refractory carrier has a plurality of parallel exhaust flow passages extending therethrough and defined by pas¬ sage walls on which the catalytic material is coated, and the catalytic moiety comprises platinum and is present in a quantity sufficient to provide from about 5 to 60 g/ft 3 of platinum. 13. The catalyst composition of claim 9 wherein the refractory carrier has a plurality of parallel exhaust flow passages extending therethrough and defined by pas¬ sage walls on which the catalytic material is coated, and the catalytic moiety comprises from about 2 to 60 g/ft. platinum and from about 5 to 50 g/ft iron. 14. The catalyst composition of claim 2, claim 3 or claim 4 wherein the zeolite is disposed in a discrete lay¬ er which is overlain by one or more discrete layers con¬ taining the alumina and the ceria. 15. The catalyst composition of claim 1, claim 2, claim 3 or claim 4 wherein the refractory carrier has a plurality of parallel exhaust flow passages extending therethrough and defined by passage walls on which the catalytic material is coated, and the ceria and alumina each has a BET surface area of from about 25 m /g to 200 m 2 /g. 16. The catalyst composition of claim 1, claim 2, claim 3 or claim 4 wherein the refractory carrier has a plurality of parallel exhaust flow passages extending therethrough and defined by passage walls on which the catalytic material is coated, and further comprising dis¬ persed platinum carried on the catalytic material in an amount of from about 0.1 to about 60 g/ft 3 . 17. The catalyst composition of claim 16 wherein the dispersed platinum is present in the amount of from about 0.1 to 5 g/ft j . 18. The catalyst composition of claim 15 wherein the refractory carrier has a plurality of parallel exhaust flow passages extending therethrough and defined by pas¬ sage walls on which the catalytic material is coated, and at least a catalytically effective amount of the dispersed platinum is carried on the ceria. 19. The catalyst composition of claim 1, claim 2, claim 3 or claim 4 wherein the refractory carrier has a plurality of parallel exhaust flow passages extending therethrough and defined by passage walls on which the catalytic material is coated, and further comprising dis¬ persed palladium carried on the catalytic material in a quantity of from about 0.1 to 200 g/ft 3 . 20. The catalyst composition of claim 19 wherein the dispersed palladium is present in an amount of from about 20 to 120 g/ft ' . 21. A method for treating a diesel engine exhaust stream containing a volatile organic fraction comprises contacting the stream with a catalyst composition under oxidizing conditions including a temperature high enough to catalyze oxidation of at least some of the volatile or¬ ganic fraction, the catalyst composition comprising a cat¬ alytically effective amount of ceria having a BET surface area of at least about 10 m 2 /g and a catalytically effec¬ tive amount of a zeolite. 22. The method of claim 21 wherein the catalyst com¬ position further comprises a catalytically effective amount of alumina having a BET surface area of at least about 10 m ' /g. 23. The method of claim 21 wherein the zeolite com¬ prises a three-dimensional zeolite characterized by pore openings whose smallest cross-sectional dimension is at least about 5 Angstroms and having a silicon to aluminum atomic ratio of greater than 5. 24. The method of claim 21 wherein the zeolite com¬ prises Beta zeolite. 25. The method of claim 21 wherein the zeolite is selected from the group consisting of Y-zeolite, pentasil, Mordenite and mixtures thereof. 26. The method of claim 23, claim 24 or claim 25 fur¬ ther including a catalytically effective amount of alumina having a BET surface area of at least about 10 m 2 /g. 27. The method of claim 26 wherein the zeolite com¬ prises from about 10 to 90 percent by weight, the alumina comprises from about 60 to 5 percent by weight, and the ceria comprises from about 60 to 5 percent by weight, of the combined weight of the zeolite, the alumina and the ceria. 28. The method of claim 21 or claim 22 wherein the zeolite is doped with a catalytic moiety selected from the group consisting of one or more of hydrogen, platinum, rhodium, palladium, ruthenium, osmium, iridium, copper, iron, nickel, chromium and vanadium. 29. The method of claim 28 wherein the zeolite is doped with the catalytic moiety by ion-exchanging the zeo¬ lite with a cationic catalytic moiety. 30. The method of claim 28 wherein the catalytic moi¬ ety comprises one or both of platinum and iron. 31. The method of claim 21 or claim 22 wherein the refractory carrier has a plurality of parallel exhaust flow passages extending therethrough and defined by pas¬ sage walls on which the catalytic material is coated, and the catalytic moiety comprises platinum and is present in a quantity sufficient to provide about 5 to 60 g/ft 3 of platinum. 32. The method of claim 22 wherein the zeolite is disposed in a discrete layer which is overlain by one or more discrete layers containing the alumina and the ceria. 33. The method of claim 22 wherein the ceria and the alumina each has a BET surface area of from about 25 m /g to 200 m 2 /g. 34. The method of claim 21 or claim 22 wherein the refractory carrier has a plurality of parallel exhaust stream flow passages extending therethrough and defined by passage walls on which the catalytic material is coated, and the catalyst material further comprises dispersed platinum carried thereon in an amount of from about 0.1 to 60 g/ft . 35. The method of claim 34 wherein the dispersed platinum is present in the amount of from about 0.1 to 5 g/ft 3 . 36. The method of claim 34 wherein at least a cata¬ lytically effective amount of the dispersed platinum is carried on the ceria. 37. The method of claim 21 or claim 22 wherein the temperature of the exhaust stream initially contacted with the catalyst composition is from about 100°C to 800°C. 38. The method of claim 21 or claim 22 wherein the refractory carrier has a plurality of parallel exhaust stream flow passages extending therethrough and defined by passage walls on which the catalytic material is coated, and the catalytic material further comprises dispersed palladium carried thereon in the amount of from about 0.1 to 200 g/ft 3 .

Inventors: ADOMAITIS JOHN R, DEEBA MICHEL, FARRAUTO ROBERT J, VOSS KENNETH E, YAVUZ BULENT O, ADOMAITIS, JOHN, R., DEEBA, MICHEL, FARRAUTO, ROBERT, J., VOSS, KENNETH, E., YAVUZ, BULENT, O.
Applicants: ENGELHARD CORP, ENGELHARD CORPORATION
Application year: 1994
Main classification: B01D53
IPC codes: B01D53, B01J29, B01J37, F02B3